It is important to accurately and reliably characterize the occupant of a vehicle seat in order to properly control the operation of air bags and other pyrotechnically deployed restraints. For example, the deployment of such restraints is generally allowed when the seat is occupied by an adult, but suppressed when the seat is occupied by a portable infant seat or child seat (referred to herein collectively as child seats). A common way of characterizing an occupant is to equip the vehicle seat with one or more weight or force responsive sensors, and to compare the sensed weight or force with calibrated thresholds. However, similar weight or force measurements occur for both an adult occupant and a child seat that is placed on the vehicle seat and cinched down with a seat belt anchored to the vehicle floor.
The U.S. Pat. No. 6,246,936 to Murphy et al. teaches that it is possible to distinguish between an adult occupant and a cinched child seat by calculating a variance of the measured occupant weight during movement of the vehicle. If the variance is below a threshold for a predetermined interval, the occupant is characterized as a child seat, since a tightly cinched seat belt severely restricts variance. If the variance exceeds the threshold for a predetermined interval, the occupant is characterized as an adult. Murphy et al. also disclose that the frequency of the measured occupant weight can be used as a correlative factor, as the frequency is higher for a cinched child seat than for a normally seated adult. In a similar vein, the U.S. Pat. No. 6,542,802 to Gray et al. teaches normalization of the measured weight variance based on a measure of the vehicle's vertical acceleration in order to compensate for disturbances such as driving on a rough road. Since the use of correlative measurements can significantly increase system cost, what is desired is a way of reliably characterizing the seat occupant with a single seat sensor.